The novel 23-membered tricyclo-macrolide, FK-506, and related compounds isolated and characterized by Tanaka, Kuroda, and co-workers, see J. Am. Chem. Soc., 109, pp. 5031, 1987, and EPO Pub. No. 0,184,162, have been shown to possess exceptional immunosuppressive activity. The potential usefulness of ##STR1## these compounds in bone marrow and organ transplantation, in the treatment of autoimmune diseases, in infectious disease treatment has been established.